The present invention relates to an oil scraper piston ring which is used in an internal combustion engine.
The oil scraper piston ring 1 (which will hereinafter be referred to as "oil scraper ring") is mounted in a piston ring groove 3 which is formed in a piston 2, as shown in FIG. 1. The piston 2 is mounted in a cylinder 4 which is processed to be perfectly circular. Compression rings 5 are mounted in piston ring grooves 31, which are provided above the oil scraper ring 1.
The primary purposes of the oil scraper ring are, for example, to act as a seal so that the lubricating oil does not enter the combustion chamber, and to scrape down the excessive lubricating oil which resides on the inner wall of the cylinder and return it to the oil pan thereby always producing a suitable lubricating oil film on the inner wall of the cylinder. Hence, what is most necessary for an oil scraper ring is that it be able to slide while it is in close contact with the inner wall of the cylinder.
Under these circumstances, many studies and developments have been made with respect to oil scraper rings. As a result of these studies there has widely been used an oil scraper ring of the integral type, as shown in FIG. 2, wherein a coiled expander 6 is fitted into a groove 13. The groove 13 is formed in the inner peripheral surface of an oil scraper ring of the integral type in which an oil window 12 is formed in an oil scraper ring proper 11.
The oil scraper ring of the integral type is manufactured so as to present the configuration of a cam having a free engaging gap in its unstressed state (i.e. with no external forces applied to the ring), and to present the configuration of a perfect circle only when it is mounted in a circular cylinder. In other words, when it is mounted on the piston and inserted into the cylinder so as to close the gap in the ring, the tension of both the oil scraper ring itself and that of the coiled expander may together press against the inner wall surface of the cylinder.
However, in the oil scraper ring of the integral type which is constituted so as to present the configuration of a perfect circle only when the free engaging gap is closed, there are several problems which are mentioned below. As a result of the study by the present inventor, it is clear that the oil scraper ring of the integral type is not preferable, because the oil scraper ring of the integral type does not sufficiently serve the primary purposes of an oil scraper ring.
In the usual manufacturing of the prior art oil scraper ring, the outer peripheral cam shape of the oil scraper ring is first formed, and the engaging portion is then cut apart so as to provide the free engaging gap. After that, the inner peripheral cutting is accomplished, followed in order, by the outer peripheral finishing, the formation of the outer peripheral groove, the formation of the oil window and the engaging portion finishing. All of the processing steps are effected with the free engaging gap closed, except for the engaging portion finishing. Therefore, it is difficult to obtain an oil scraper ring having an outer peripheral surface configuration which conforms exactly with the inner wall of a circular cylinder.
In addition, because the inner peripheral cutting takes place after the cutting of the free engaging gap portion, the outer peripheral finishing and the outer peripheral groove processing become intermittent cutting, because a gap has already been cut in the periphery. This causes many problems, for example, in that the life of the cutting tool becomes short and the cutting speed cannot be enhanced.